To make informed prevention resource allocation decisions, public health decision makers require detailed information about the cost, cost-effectiveness, and potential impact of various strategies to prevent transmission of HIV and other STDs. The Cost-Effectiveness Modeling Core supports scientific research into the cost, economic efficiency, and effectiveness of HIV/STD prevention interventions, related behavioral modification programs, and intervention dissemination and technology transfer activities. The Core provides consultation to CAIR Investigators regarding the feasibility of conducting economic efficiency or modeling studies of proposed, ongoing, or completed HIV prevention and dissemination/technology transfer interventions. Core Scientists identify appropriate economic evaluation or mathematical modeling techniques for assessing the cost-effectiveness ofthe intervention and collaborate with other CAIR Scientists on the design of suitable behavioral measurement and cost data collection instruments to support economic evaluation research and modeling studies. Core Scientists take a lead role in performing these analyses. The Core's support activities include prospective cost data elicitation during the course of an ongoing intervention trial; retrospective data collection after a trial is complete; mathematical modeling to quantify intervention effectiveness in epidemiologically-meaningful units (e.g., HIV infections averted or expected number of secondary infections); and/or cost-effectiveness, cost-utility, or other economic analyses to determine the overall economic efficiency of HIV prevention interventions.